Array Sum

Given an array of 32-bit signed integers, compute and return their sum using AVX2 intrinsics.

Function Signature

#include <immintrin.h>
#include <cstdint>

int32_t array_sum(const int32_t* arr, int n);

Parameters:

arr — pointer to an array of n signed 32-bit integers, guaranteed 32-byte aligned
n — number of elements, guaranteed to be a multiple of 8 and at least 8

Returns: the sum of all elements in the array (guaranteed to fit in int32_t)

Example

Input:  [1, 2, 3, 4, 5, 6, 7, 8]
Output: 36

Constraints

8 ≤ n ≤ 1,000,000
n is always a multiple of 8
All elements are in range [-1000, 1000]
The total sum fits in a 32-bit signed integer
arr is 32-byte aligned

Notes

Your solution should use AVX2 intrinsics to process 8 integers at a time. A scalar solution will produce correct results but will not achieve a meaningful speedup.

Useful Intrinsics

Intrinsic	Description
`_mm256_setzero_si256()`	Create a zero vector
`_mm256_load_si256(ptr)`	Load 256 bits from aligned memory
`_mm256_add_epi32(a, b)`	Add packed 32-bit integers
`_mm256_extracti128_si256(v, 1)`	Extract high 128-bit lane
`_mm256_castsi256_si128(v)`	Cast 256-bit to low 128-bit (free)
`_mm_add_epi32(a, b)`	Add packed 32-bit integers (128-bit)
`_mm_hadd_epi32(a, b)`	Horizontal add of packed 32-bit integers
`_mm_extract_epi32(v, idx)`	Extract a 32-bit integer